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Old 01-01-2014, 09:11 AM   #1
caphair
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Default Sega Genesis 1 - No mono audio output

Not sure if anyone will be able to help me with this, found my old sega genesis and hooked it up, video is fine but I have no audio out from either the RF or SCART connectors but the front headphone jack works.

I found a schematic but I'm not too familiar with how audio works, it looks like the mono output goes through a LM358, could this be bad?

The attached image doesn't zoom, here's a direct link to it:
http://cgfm2.emuviews.com/img/mega3.gif
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Last edited by caphair; 01-01-2014 at 09:13 AM..
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Old 01-01-2014, 09:26 AM   #2
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Default Re: Sega Genesis 1 - No mono audio output

First thing I would check is the switch in the headphone jack. This can go bad sometimes and cause issues like this. The switch contacts get oxidised and don't make connection very well.

Is it possible that you could get that schematic picture in a higher resolution? Because it is hard - no, impossible to read the part numbers and values
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Old 01-01-2014, 09:40 AM   #3
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Default Re: Sega Genesis 1 - No mono audio output

I posted a direct link to it which is much clearer/bigger.

here it is again:

http://cgfm2.emuviews.com/img/mega3.gif


How would I clean the switch? Can I just spray something in or does it need to be taken apart? Thanks
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Old 01-01-2014, 09:43 AM   #4
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Default Re: Sega Genesis 1 - No mono audio output

Looking at the schematic, it doesn't look like the headphone jack has a switch, looks like there are two sound chips. One for stereo going to headphone and another going to mono.

I could be wrong?
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Old 01-01-2014, 09:47 AM   #5
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Default Re: Sega Genesis 1 - No mono audio output

Yeah, my apologies.

There doesn't appear to be a switch in the headphones jack.

Well, yes, the LM358 could be bad, or the chip IC13 could be bad.

You probably could try connecting an audio amplifier and speaker to the output of the LM358, and see if you can get any sound. If you can, then I'd suspect IC13 is bad. However, perhaps the LM358 may be passing audio, but not enough for IC13. Check the feedback resistor in the LM358 amp circuit and make sure it is ok.
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Old 01-01-2014, 09:57 AM   #6
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Default Re: Sega Genesis 1 - No mono audio output

Quote:
Originally Posted by ben7 View Post
Yeah, my apologies.

There doesn't appear to be a switch in the headphones jack.

Well, yes, the LM358 could be bad, or the chip IC13 could be bad.

You probably could try connecting an audio amplifier and speaker to the output of the LM358, and see if you can get any sound. If you can, then I'd suspect IC13 is bad. However, perhaps the LM358 may be passing audio, but not enough for IC13. Check the feedback resistor in the LM358 amp circuit and make sure it is ok.
Are you referring to R74 33K? It reads 32K

I'm not exactly sure how I could connect an amp and speaker to it's output. Any voltage readings I could try?

Also I looked up the LM358 and it's a two stage op amp, looks like only one stage is being used, could I try connecting inputs to it's second stage to see if I get audio?
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Old 01-01-2014, 11:36 AM   #7
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Default Re: Sega Genesis 1 - No mono audio output

inject at c30.bet its open.had a few in here for that.
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Old 01-01-2014, 11:47 AM   #8
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Default Re: Sega Genesis 1 - No mono audio output

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Originally Posted by kc8adu View Post
inject at c30.bet its open.had a few in here for that.
Already tried swapping that cap and still no audio, though I only had on hand a 10uf 25v one and original is 10uf 50v. Not sure if that matters? It was for a quick test
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Old 01-02-2014, 05:40 AM   #9
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Default Re: Sega Genesis 1 - No mono audio output

Can someone help me understand how this circuit works, are LM358 and IC12 both amplifiers for the sound coming from IC11?

If so, why does the output of LM358 go into IC13 rather than just straight out to RF/SCART outputs?
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Old 01-02-2014, 08:54 AM   #10
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Default Re: Sega Genesis 1 - No mono audio output

Yes, they are both amplifiers. IC12 is the headphone amplifier, and the LM358 produces an amplified mono signal. The datasheet for IC13 (CXA1145) says that it only has a buffer inside for the audio section, so it should pass the input signal directly through without altering it.

C31 could be bad as well.

Last edited by lti; 01-02-2014 at 08:55 AM..
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Old 01-02-2014, 09:01 AM   #11
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Default Re: Sega Genesis 1 - No mono audio output

Quote:
Originally Posted by lti View Post
Yes, they are both amplifiers. IC12 is the headphone amplifier, and the LM358 produces an amplified mono signal. The datasheet for IC13 (CXA1145) says that it only has a buffer inside for the audio section, so it should pass the input signal directly through without altering it.

C31 could be bad as well.
Awesome. Thanks. By buffer what does that mean exactly? Also anyway I can pinpoint where audio is lost with just a multimeter?

C31 looks like it's only before the SCART connection which also doesn't produce audio.

Could I run some wires from output of LM358 to the audio pin of RF module bypassing IC13 to see if I get audio out?
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Old 01-02-2014, 09:05 AM   #12
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Default Re: Sega Genesis 1 - No mono audio output

IC13 CXA1145 spec:
http://pdf1.alldatasheet.com/datashe.../CXA1145P.html
http://console5.com/wiki/CXA1145
The LM358 is probably does not have enough power to drive the modulator module and the SCART so the output of the LM358 is fed into the buffer amp inside the IC13. LM358 is used as a mixer for Left and Right signals.
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Old 01-02-2014, 09:17 AM   #13
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Default Re: Sega Genesis 1 - No mono audio output

Quote:
Originally Posted by budm View Post
IC13 CXA1145 spec:
http://pdf1.alldatasheet.com/datashe.../CXA1145P.html
http://console5.com/wiki/CXA1145
The LM358 is probably does not have enough power to drive the modulator module and the SCART so the output of the LM358 is fed into the buffer amp inside the IC13. LM358 is used as a mixer for Left and Right signals.
Cool I get it now. So I couldn't simply try to bypass ic13 to see if I get sound and determine it's the problem?
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Old 01-02-2014, 10:00 AM   #14
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Default Re: Sega Genesis 1 - No mono audio output

You can lift C30 positive leg off the board, C31 positive leg off the board, then tie those two positive legs of those 2 caps together and see if you will get sound on the SCART.
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Old 01-02-2014, 10:07 AM   #15
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Default Re: Sega Genesis 1 - No mono audio output

Quote:
Originally Posted by budm View Post
You can lift C30 positive leg off the board, C31 positive leg off the board, then tie those two positive legs of those 2 caps together and see if you will get sound on the SCART.
By doing that will it boost the sound from LM358 enough to output sound?
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Old 01-02-2014, 02:59 PM   #16
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Default Re: Sega Genesis 1 - No mono audio output

We just want to verify that LM358 is putting out audio or if it is dead, you should hear the sound, if not, then LM358 is bad not the buffer amp problem in the IC13. Doing as I indicated above will bypass the IC13 to see where the problem is.
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Old 01-02-2014, 03:58 PM   #17
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Default Re: Sega Genesis 1 - No mono audio output

Quote:
Originally Posted by budm View Post
We just want to verify that LM358 is putting out audio or if it is dead, you should hear the sound, if not, then LM358 is bad not the buffer amp problem in the IC13. Doing as I indicated above will bypass the IC13 to see where the problem is.
Ok I'm just trying to learn as I go along. In this case what would tying those two caps do as oppose to just going straight from LM538's output to SCART?

I'll try it regardless just curious.

Last edited by ohmmy; 01-02-2014 at 04:05 PM..
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Old 01-02-2014, 04:14 PM   #18
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Default Re: Sega Genesis 1 - No mono audio output

I have you lift the leg for easy way to try out, If it is SMD cap, then that will be problem, we just need to take the output of the LM358 and feed it to coupling cap and to not allow the DC at the output of the LM358 feeding into the receiving circuit, at the same time we must disconnect the input and output of IC13 from being connected to the circuit. The output pin of IC 13 has very low impedance so you cannot leave it connected so if you just jump the output of the LM358 to the SCART, the output of IC13 will load down the circuit.

Last edited by budm; 01-02-2014 at 04:17 PM..
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Old 01-03-2014, 10:07 AM   #19
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Default Re: Sega Genesis 1 - No mono audio output

Ok, tied them two caps together and still no audio from SCART.

So I can conclude the LM358 is defective?
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Old 01-03-2014, 06:50 PM   #20
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Default Re: Sega Genesis 1 - No mono audio output

Sounds like it if you do not even hear anything at all. Another test is to check the DC voltage at the output pin of the LM358, it should be around 1/2 of the Vcc power supply voltage.

Last edited by budm; 01-03-2014 at 06:52 PM..
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